Reading quickly through FMP's thread, it doesn't look like anyone else has had map problems. You might have caught a faulty version, though I doubt it, or something much more strange is going on. I'll download it and check out with my regular copy of OXCE+.
In the meantime, if what's available now is a later version, give it an upgrade anyway, and see if that also solves the issue. At a low chance, but, it could be weirdness in how your OXC version interacts. I'm kind of grasping now because it's so strange, and I don't really have a clue. All I mentioned before was regular troubleshooting steps.
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Edit:
Well, I took a look.
Using:
OXCE+: 3.10a 2018-1-31
FMP: 1.9.8 (also without)
Resolution: 640x480
Geoscape Scaling: Full Display
Filter: Scale
I have made these screenshots.
000, 001 are with FMP.
002, 003 are without FMP.
It looks like rather it's either undocumented map changes, or we just haven't read through all the changes made in FMP. Either way, looks like just a change to the Geoscape map.